1. Field of the Invention
The present invention relates to semiconductor packages, and more particularly to packages in which the various parts such as the flange, the window frame and the leads are joined together using a filler metal.
2. History of the Prior Art
It is known in the art to provide semiconductor packages in which one or more semiconductor dies are mounted on a heatsink flange within an opening in a window frame which mounts and insulates a plurality of leads. The dies may be of the LDMOS (lateral diffusion metal oxide semiconductor) type and the package of the type for packaging LDMOS power transistors. The window frame serves to mount the leads on the semiconductor package and insulate the leads from the heatsink flange and other portions of the package. The window frame has an opening therein which surrounds the semiconductor dies. The dies are electrically coupled to the leads such as by wire bonds.
In semiconductor packages of the type described, the component parts thereof, including the flange, the window frame and the leads, are typically joined together using a filler metal. Typically, such filler metals are silver based. The filler metal acts to bind the flange to the window frame and the leads to the window frame. An example of a silver-based filler metal commonly used to bind together the parts of the semiconductor package is 72Ag28Cu (CuSil).
Silver-based filler metals such as CuSil are effective in binding the flange to the window frame and the leads to the window frame. Such metals can withstand the high temperatures and other conditions associated with the manufacture of the semiconductor package, and continue to bind the parts together during subsequent use of the package. However, problems may occur during subsequent use of the semiconductor package, particularly where the package is not contained within a hermetically sealed enclosure or with a hermetic lid. The filler metal provides an exposed silver source. Moisture can seep into the package and condense along the dielectric surface of the window frame between the filler metal and the flange and the leads. With a potential difference applied between the negative flange and the positive leads, silver migration occurs. Eventually, such silver migration may bridge and create an electrical short between the positive leads and the negative flange. If a continuous layer of moisture forms between the leads and the flange, ionized silver travels along the condensed water covering the dielectric window frame and deposits at the flange in pure metal form. Eventually, the silver deposits bridge the flange and the leads to create an electrical short.
Silver migration has long been a problem for the electronics industry, often requiring changes to current and future product designs. One way to ensure that silver migration does not occur is to use a filler metal which contains no silver. Other alternatives involve the use of adhesives, conformal coatings, and additives such as Pd, Y and the like. However, adhesives and conformal coatings are usually unable to survive the high processing temperatures of 300° C. or more. Filler metals or additives which do not contain silver tend to have less than desirable properties, such as increased brittleness, high processing temperatures, and non-uniform wetting.
For this reason, CuSil is still preferred as the filler metal for most applications. Such material provides ideal electrical conductivity as well as desirable mechanical properties such as high strength, high ductility and smooth joints. However, silver migration continues to be a problem with such material.